CCBC-1630: check collection id before storing packet to pipeline

Every time check_collection_id() is invoked, the caller should ensure
that this function potentially is rewriting the packet, if it decides to
insert/update encoded collection ID.

Change-Id: I3d9964c2af04724f95912bc715c76518fba9a5e0
Reviewed-on: https://review.couchbase.org/c/libcouchbase/+/205841
Reviewed-by: Michael Reiche <michael.reiche@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>

Author: avsej

src/mc/mcreq.c

@@ -333,10 +333,11 @@ static mc_PACKET *check_collection_id(mc_PIPELINE *pipeline, mc_PACKET *packet)
 
 void mcreq_enqueue_packet(mc_PIPELINE *pipeline, mc_PACKET *packet)
 {
+    packet = check_collection_id(pipeline, packet);
+
     nb_SPAN *vspan = &packet->u_value.single;
     sllist_append(&pipeline->requests, &packet->slnode);
 
-    packet = check_collection_id(pipeline, packet);
     netbuf_enqueue_span(&pipeline->nbmgr, &packet->kh_span, packet);
     MC_INCR_METRIC(pipeline, bytes_queued, packet->kh_span.size);